Capacitive filtered feedthrough array for an implantable medical device

ABSTRACT

A capacitive filtered feedthrough assembly is formed in a solid state manner to employ highly miniaturized conductive paths each filtered by a discoid capacitive filter embedded in a capacitive filter array. A non-conductive, co-fired metal-ceramic substrate is formed from multiple layers that supports one or a plurality of substrate conductive paths and it is brazed to a conductive ferrule, adapted to be welded to a case, using a conductive, corrosion resistant braze material. The metal-ceramic substrate is attached to an internally disposed capacitive filter array that encloses one or a plurality of capacitive filter capacitor active electrodes each coupled to a filter array conductive path and at least one capacitor ground electrode. Each capacitive filter array conductive path is joined with a metal-ceramic conductive path to form a feedthrough conductive path. Bonding pads are attached to the internally disposed ends of each feedthrough conductive path, and corrosion resistant, conductive buttons are attached to and seal the externally disposed ends of each feedthrough conductive path. A plurality of conductive, substrate ground paths are formed extending through the co-fired metal-ceramic substrate between internally and externally facing layer surfaces thereof and electrically isolated from the substrate conductive paths. The capacitor ground electrodes are coupled electrically to the plurality of conductive, substrate ground paths and to the ferrule.

This application is a divisional of application Ser. No. 09/515,385,filed Mar. 1, 2000, now U.S. Pat. No. 6,414,835.

FIELD OF THE INVENTION

This invention relates to electrical feedthroughs of improved design andto their method of fabrication, particularly for use with implantablemedical devices.

BACKGROUND OF THE INVENTION

Electrical feedthroughs serve the purpose of providing an electricalcircuit path extending from the interior of a hermetically sealed caseor housing to an external point outside the case. Implantable medicaldevices (IMDs) such as implantable pulse generators (IPGs) for cardiacpacemakers, implantable cardioverter/defibrillators (ICDs), nerve,brain, organ and muscle stimulators and implantable monitors, or thelike, employ such electrical feedthroughs through their case to makeelectrical connections with leads, electrodes and sensors locatedoutside the case.

Such feedthroughs typically include a ferrule adapted to fit within anopening in the case, one or more conductor and a non-conductive hermeticglass or ceramic seal which supports and electrically isolates each suchconductor from the other conductors passing through it and from theferrule. The IMD case is typically formed of a biocompatible metal,e.g., titanium, although non-conductive ceramics materials have beenproposed for forming the case. The ferrule is typically of a metal thatcan be welded or otherwise adhered to the case in a hermetically sealedmanner.

Typically, single pin feedthroughs supported by glass, sapphire andceramic were used with the first hermetically sealed IMD cases for IPGs.As time has passed, the IPG case size has dramatically reduced and thenumber of external leads, electrodes and sensors that are to be coupledwith the circuitry of the IPG has increased. Consequently, use of therelatively large single pin feedthroughs is no longer feasible, andnumerous multiple conductor feedthroughs have been used or proposed foruse that fit within the smaller sized case opening and provide two,three, four or more conductors.

Many different insulator structures and conductor structures are knownin the art of multiple conductor feedthroughs wherein the insulatorstructure also provides a hermetic seal to prevent entry of body fluidsthrough the feedthrough and into the housing of the medical device. Theconductors typically comprise electrical wires or pins that extendthrough a glass and/or ceramic layer within a metal ferrule opening asshown, for example, in commonly assigned U.S. Pat. Nos. 4,991,582,5,782,891, and 5,866,851 or through a ceramic case as shown in thecommonly assigned '891 patent and in U.S. Pat. No. 5,470,345. It hasalso been proposed to use co-fired ceramic layer substrates that areprovided with conductive paths formed of traces and vias as disclosed,for example, in U.S. Pat. Nos. 4,420,652, 5,434,358, 5,782,891,5,620,476, 5,683,435, 5,750,926, and 5,973,906.

Such multi-conductor feedthroughs have an internally disposed portionconfigured to be disposed inside the case for connection with electricalcircuitry and an externally disposed portion configured to be disposedoutside the case that is typically coupled electrically with connectorelements for making connection with the leads, electrodes or sensors.The elongated lead conductors extending from the connector elementseffectively act as antennae that tend to collect stray electromagneticinterference (EMI) signals that may interfere with normal IMDoperations. At certain frequencies, for example, EMI can be mistaken fortelemetry signals and cause an IPG to change operating mode.

This problem has been addressed in certain of the above-referencedpatents by incorporating a capacitor structure upon the internallyfacing portion of the feedthrough ferrule coupled between eachfeedthrough conductor and a common ground, the ferrule, to filter outany high frequency EMI transmitted from the external lead conductorthrough the feedthrough conductor. The feedthrough capacitors originallywere discrete capacitors but presently can take the form of chipcapacitors that are mounted as shown in the above-referenced '891, '435,'476, and '906 patents and in further U.S. Pat. Nos. 5,650,759,5,896,267 and 5,959,829, for example. Or the feedthrough capacitors cantake the form of discrete discoidal capacitive filters or discoidalcapacitive filter arrays as shown in commonly assigned U.S. Pat. Nos.5,735,884, 5,759,197, 5,836,992, 5,867,361, and 5,870,272 and furtherU.S. Pat. Nos. 5,287,076, 5,333,095, 5,905,627 and 5,999,398.

These patents disclose use of discoidal filters and filter arrays inassociation with conductive pins which are of relatively large scale anddifficult to miniaturize without complicating manufacture. It isdesirable to further miniaturize and simplify the fabrication of themulti-conductor feedthrough assembly

Although feedthrough filter capacitor assemblies of the type describedabove have performed in a generally satisfactory manner, the manufactureand installation of such filter capacitor assemblies has been relativelytime consuming and therefore costly. For example, installation of thediscoidal capacitor into the small annular space between the terminalpin and ferrule as shown in a number of these patents can be a difficultand complex multi-step procedure to ensure formation of reliable, highquality electrical connections.

Other problems have arisen when chip capacitors have been coupled toconductive trace and via pathways of co-fired multi-layer metal-ceramicsubstrates disclosed in the referenced '652, '358, '891, '476, '435,'926, and '906 patents. The conductive paths of the feedthrough arraysand attached capacitors suffer from high inductance which has the effectof failing to attenuate EMI and other unwanted signals, characterized as“poor insertion loss”.

A high integrity hermetic seal for medical implant applications is verycritical to prevent the ingress of body fluids into the IMD. Even asmall leak rate of such body fluid penetration can, over a period ofmany years, build up and damage sensitive internal electroniccomponents. This can cause catastrophic failure of the implanted device.The hermetic seal for medical implant (as well as space and military)applications is typically constructed of highly stable alumina ceramicor glass materials with very low bulk permeability. The above-describedfeedthroughs formed using metal-ceramic co-fired substrates, however,have not been hermetic because the metal component of the substratecorrodes in body fluids, and the substrates have cracked from stressesthat developed from brazing and welding processes.

Withstanding the high temperature and thermal stresses associated withthe welding of a hermetically sealed terminal with a premounted ceramicfeedthrough capacitor is very difficult to achieve with the '551, '095and other prior art designs. The electrical/mechanical connection to theoutside perimeter or outside diameter of the feedthrough capacitor has avery high thermal conductivity as compared to air. The welding operationtypically employed in the medical implant industry to install thefiltered hermetic terminal into the IMD case opening can involve awelding operation in very close proximity to this electrical/mechanicalconnection area. Accordingly, in the prior art, the ceramic feedthroughcapacitors are subjected to a dramatic temperature rise. Thistemperature rise produces mechanical stress in the capacitor due to themismatch in thermal coefficients of expansion of the surroundingmaterials.

In addition, in the prior art, the capacitor lead connections must be ofvery high temperature materials to withstand the high peak temperaturesreached during the welding operation (as much as 500 C. °). A similar,but less severe, situation is applicable in military, space andcommercial applications where similar prior art devices are solderedinstead of welded by the user into a bulkhead or substrate. Many ofthese prior art devices employ a soldered connection to the outsideperimeter or outside diameter of the feedthrough capacitor. Excessiveand unevenly applied soldering heat has been known to damage such priorart devices. Accordingly, there is a need for a filter capacitor andfeedthrough array in a single assembly that addresses the drawbacksnoted above in connection with the prior art.

In particular, a capacitive filtered feedthrough array is needed that issubjected to far less temperature rise during the manufacture thereof.Moreover, such an improvement would make the assembly relatively immuneto the aforementioned stressful installation techniques.

Moreover, a capacitive filtered feedthrough array is needed which is ofsimplified construction, utilizing a straightforward and uncomplicatedassembly, that can result in manufacturing cost reductions. Of coursethe new design must be capable of effectively filtering out undesirableEMI. The present invention fulfills these needs and provides otherrelated advantages.

SUMMARY OF THE INVENTION

A capacitive filtered feedthrough assembly is formed in accordance withthe present invention in a solid state manner to employ highlyminiaturized conductive paths each filtered by a discoid capacitivefilter embedded in a capacitive filter array. A non-conductive, co-firedmetal-ceramic substrate is formed from multiple layers that supports oneor a plurality of substrate conductive paths and it is brazed to aconductive ferrule, adapted to be welded to a case, using a conductive,corrosion resistant braze material. The metal-ceramic substrate isattached to an internally disposed capacitive filter array that enclosesone or a plurality of capacitive filter capacitor active electrodes eachcoupled to a filter array conductive path and at least one capacitorground electrode. Each capacitive filter array conductive path is joinedwith a metal-ceramic conductive path to form a feedthrough conductivepath. Bonding pads are attached to the internally disposed ends of eachfeedthrough conductive path, and corrosion resistant, conductive buttonsare attached to and seal the externally disposed ends of eachfeedthrough conductive path. Each capacitor ground electrode iselectrically coupled with the ferrule.

Preferably, a plurality of such feedthrough conductive paths are formed,and each capacitive filter comprises a plurality of capacitor active andground electrodes, wherein the capacitor ground electrodes areelectrically connected in common.

Moreover, preferably, a plurality of conductive, substrate ground pathsare formed extending through the co-fired metal-ceramic substratebetween internally and externally facing layer surfaces thereof andelectrically isolated from the substrate conductive paths. The capacitorground electrodes are coupled electrically to the plurality ofconductive, substrate ground paths and to the ferrule.

In addition, preferably, the capacitive filter array conductive pathsare formed by solder filling holes extending through the filter arraysubstrate between internally and externally facing array surfacesthereof. The application of the solder also joins the externally facingarray surface with the internally facing metal-ceramic substrate layersurface and electrically joins the capacitive filter array conductivepaths with the metal-ceramic conductive paths to form the feedthroughconductive paths.

Utilization of an internally grounded, metal-ceramic substrate providinga plurality of conductive substrate paths in stacked, aligned, relationto a capacitive filter array as disclosed herein provides a number ofadvantages:

A hermetic seal is achieved by brazing a co-fired metal-ceramicsubstrate with low permeability to a metallic ferrule. The inventiveferrule-substrate braze joint design minimizes the tensile stresses inthe co-fired substrate, thus preventing cracking of the co-firedsubstrate during brazing and welding. In addition, the ferrule has athin flange which minimizes stress applied to the co-fired substrateduring welding. Corrosion of the co-fired metal phase of the substrateis prevented by protecting the exposed metal vias and pads withcorrosion resistant metallizations and braze materials.

Because the capacitive filter array is displaced from the ferrule andsupported by the metal-ceramic substrate, the heat imparted to theferrule flange during welding causes minimal temperature elevation ofthe capacitive filter array, and does not cause damage to it.

The attachment of the conductive paths of the outward facing capacitivefilter surface to the metallized layers of the inward facing surface ofthe metal-ceramic substrate using reflow soldering provides secureattachment and low resistance electrical connection and simplifiesmanufacturing. The use of conductive epoxy compounds for adhesion isthereby avoided. Conductive epoxy adhesion layers can bridge thenon-conductive ceramic between adjacent conductive paths and causeelectrical shorts. And voids can occur in bridging the conductive pathsof the metal-ceramic substrate and the capacitive filter elements.

The reflow soldering attachment of the of the conductive paths of theoutward facing capacitive filter surface to the metallized layers of theinward facing surface of the metal-ceramic substrate also isadvantageous in that the solder flow takes place in an oven underuniformly applied temperature to the entire assembly, thereby avoidingdamage that can be caused in hand soldering such parts together.

The capacitor ground electrodes of the discoidal capacitors of thecapacitive filter array are electrically coupled together and throughthe plurality of substrate ground paths of the metal-ceramic substrateand then through the braze to the ferrule. The plurality of substrateground paths are selected in total cross-section area to provide a totalground via cross-section area that minimizes the inductance of thefiltered feedthrough assembly, resulting in favorable insertion loss ofEMI and unwanted signals.

Size of the feedthrough is decreased by eliminating the pins, the pinbraze joints, and the welds between the pins. The pin-to-pin spacing oftwo single pin or unipolar feedthroughs is typically on the order of0.125 inches. The above-described capacitive filtered feedthrough arrayprovides a spacing of 0.050 inches between adjacent conductive paths.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description of the preferred embodiment of theinvention when considered in connection with the accompanying drawings,in which like numbered reference numbers designate like parts throughoutthe figures thereof, and wherein:

FIG. 1 is a perspective view of the filtered feedthrough assembly of thepresent invention adapted to be fitted into an opening of a case of ahermetically sealed electronic device showing the externally disposedportion configured to be disposed outside and face outwardly from thecase;

FIG. 2 is a perspective view of the filtered feedthrough assembly of thepresent invention adapted to be fitted into an opening of a case of ahermetically sealed electronic device showing the internally disposedportion configured to be disposed inside the case and face inward;

FIG. 3 is a plan view looking toward the internally disposed portion ofthe filtered feedthrough assembly of the present invention;

FIG. 4 is a cross-section side view of the filtered feedthrough assemblytaken along lines 4—4 of FIG. 3;

FIG. 5 is an expanded end portion of the cross-section view of FIG. 4

FIG. 6 is a cross-section end view of the filtered feedthrough assemblytaken along lines 6—6 of FIG. 3;

FIG. 7 is an exploded view of the components of the filtered feedthroughassembly of FIGS. 1-6;

FIG. 8 is a perspective view of the filtered feedthrough assembly of thepresent invention fitted into an opening of a half portion of the caseof a hermetically sealed electronic device showing the externallydisposed portion outside the case;

FIG. 9 is a perspective view of the filtered feedthrough assembly of thepresent invention fitted into the opening of the case half portion ofFIG. 7 showing the internally disposed portion inside the case andelectrically connected to an electrical component;

FIG. 10 is a flow chart illustrating the steps of fabricating themulti-layer, co-fired metal-ceramic substrate adapted to be brazed withthe capacitive filter array formed in the steps of FIG. 11, the ferrule,and other components in the steps of FIG. 12;

FIG. 11 is a flow chart illustrating the steps of fabricating thecapacitive filter array adapted to be brazed with the co-firedmetal-ceramic substrate formed in the steps of FIG. 10, the ferrule, andother components in the steps of FIG. 12; and

FIG. 12 is a flow chart illustrating the steps of fabricating thefiltered feedthrough assembly from the capacitive filter array formed inthe steps of FIG. 11, the co-fired metal-ceramic substrate formed in thesteps of FIG. 10, the ferrule, and other components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1-7 depict the filtered feedthrough assembly 10 adapted to befitted into an opening 204 of a case 202 of a hermetically sealedelectronic device 200 as shown in FIGS. 8 and 9 and manufactured inaccordance with the flow chart steps of FIGS. 10-12. The feedthroughassembly 10 has an internally disposed portion 12 configured to bedisposed inside the case 202 and an externally disposed portion 14configured to be disposed outside the case 202.

The filtered feedthrough assembly 10 shown in FIGS. 1-9 comprises aelectrically conductive ferrule 20 having a ferrule wall 22 with aninner wall surface 24 defining a centrally disposed ferrule opening 30and extending between opposed internal and external sides 26 and 28.When ferrule 20 is fitted into the case opening 204, the internallyfacing side 26 is adapted face toward the inside of the case 202, andthe externally facing side 28 is adapted face toward the exterior of thecase 202. The electrically conductive ferrule 20 further comprises arelatively thin welding flange 32 extending outwardly of the ferrulewall 22 away from the ferrule opening 30 for a predetermined distancedefining a flange width FW. The flange 32 is formed to have a relativelythin flange thickness FT for absorbing stress caused by thermal weldingenergy applied to the ferrule 20 in the process of welding the flange 32to the case 202 around the case opening 204 as shown in FIGS. 8 and 9.

The ferrule 20 is preferably formed of a conductive material selectedfrom the group consisting of niobium, titanium, titanium alloys such astitanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium,tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium,ruthenium, palladium, silver, and alloys, mixtures and combinationsthereof. Niobium is the optimal material for forming the ferrule 20because it has a coefficient of thermal expansion (CTE) that iscompatible with the CTE of the substrate 40 so that heat-induced duringbrazing of the metal-ceramic substrate edge to the ferrule inner wallsurface 24 does not damage the substrate 40.

The multi-layer, co-fired metal-ceramic substrate 40 shown in detail inFIGS. 5 and 7 has an internally facing major surface or side 42 and anexternally facing major surface or side 44 that are joined by a commonsubstrate edge 46. The substrate 40 is dimensioned and shaped to fitwithin the ferrule opening 30 with the common substrate edge 46 in closerelation to the ferrule inner wall surface 24. The common substrate edge46 is brazed to the ferrule inner wall surface 24 using asubstrate-ferrule braze joint 48.

The metal-ceramic substrate 40 is formed of a plurality of planarceramic layers 52, 54, 56, 58 and 60. Each ceramic layer is shaped in agreen state to have a layer thickness and a plurality of via holesextending therethrough between an internally facing layer surface and anexternally facing layer surface. The co-fired metal-ceramic substrateceramic material comprises one of the group consisting essentially ofalumina, aluminum nitride, beryllium oxide, silicon dioxide, andglass-ceramic materials that has a CTE compatible with the CTE of thematerial of the ferrule.

A plurality (nine in the depicted example) of conductive paths, e.g.path 50 shown in FIGS. 5 and 6, extend through the layers 52-60 ofco-fired metal-ceramic substrate 40 and are electrically isolated fromone another by the ceramic material. Conductive path 50 (and all theother conductive paths) comprises a plurality of electrically conductivevias 62, 64, 66, 68 extending through the plurality of layer thicknessesand a like plurality of electrically conductive traces 72, 74 and 76formed on certain of the internally or externally facing layer surfacessuch that the conductive trace 72 joins the conductive vias 62 and 64,the conductive trace 74 joins the conductive vias 64 and 66, and theconductive trace 76 joins the vias 66 and 68 to form the conductive path50. The layer holes and vias 62-68 filling them are staggered in theelongated direction of the feedthrough assembly 10 as shown in FIGS. 4and 5 but are aligned in the narrow direction as shown in FIG. 6. Theconductive vias and traces are formed of a refractory metal, e.g.,tungsten, as described further in reference to FIG. 10.

A further plurality (twenty in the depicted example) of ground paths 118each comprising substrate ground paths 118 extending through all layers52-60 spaced apart around the periphery of the co-fired metal-ceramicsubstrate 40. The ground paths 118 also comprise one or more groundtrace, e.g. ground plane traces or layers 132, 134 and 136 shown in FIG.5, extending peripherally along the substrate layer surfaces from thesubstrate ground paths 118 to the substrate edge 46. The ground trace132 assists in making electrical contact with the ground solder joint130 and with the substrate-ferrule braze joint 48. The ground traces 134and 136 extend to a metallization layer 140 formed over the substrateedge 46. The number of substrate ground paths 118 substrate ground paths118 formed in this manner is selected to provide a total ground viacross-section area that minimizes the inductance of the filteredfeedthrough assembly 10 resulting in favorable insertion loss of EMI andunwanted signals.

Each such conductive path 50 extends all the way through the substrate40 between the internally facing side 42 and the externally facing side44. On the externally facing side 44, ceramic layer 60 is formed in thegreen state with a plurality of button cavities 80 each aligned with avia 68 of layer 58. A substrate conductor pad or button 70 is fittedwithin each button cavity 80 of the layer 60 and adhered to via 68 by abutton braze joint 78 formed of gold or a nickel-gold alloy. The pads orbonding buttons 70 are preferably formed of a conductive materialselected from the group consisting of niobium, platinum or aplatinum-iridium alloy, titanium, titanium alloys such astitanium-6Al-4V or titanium-vanadium, molybdenum, zirconium, tantalum,vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium,palladium, silver, and alloys, mixtures and combinations thereof. Inthis way, a plurality of externally disposed bonding buttons 70 aresupported along the externally disposed feedthrough portion 14, and eachexternally disposed bonding button 70 is electrically conducted with anelectrically conductive path 50 of the metal-ceramic substrate 40.

A substrate conductor plated pad 82 is formed of gold or a gold alloy onthe internally facing surface of layer 52 and electrically coupled tovia 62. A solder layer 128 adheres to plated pad 82 during assembly ofthe feedthrough assembly 10 described below with reference to FIG. 12.The plurality of ceramic layers are shaped punched with holes, andprinted with the traces and vias and assembled together in the ceramicgreen state, and the assembly is then co-fired from the green state toform the substrate 40 as further described below in reference to FIG.10.

The discoidal capacitive filter array 90 is formed of a ceramiccapacitive filter array substrate 92 having an internally facing filtersubstrate side 94 and an externally facing filter substrate side 96joined by a common filter substrate edge 98. The discoidal capacitorfilter array 90 is formed with a plurality of discoidal capacitivefilters, e.g., capacitive filter 100, that are electrically connectedwith a respective one of the substrate conductive paths, e.g.,conductive path 50, and provide a filtered electrically conductive pathbetween the internally disposed bonding pad 120 and the externallydisposed button 70. The number of conductive paths so formed can varyfrom the nine that are depicted in FIGS. 1-9.

The capacitive filter array substrate 92 is preferably formed of layersof barium titanate and precious metal traces in the manner describedbelow with reference to FIG. 11. The plurality of capacitive filters 100and the filter array conductive paths 110 associated therewith areformed in and electrically isolated from one another by the ceramicmaterial and extend between the internally facing filter substrate side94 and the externally facing filter substrate side 96. Each filter arrayconductive path 110 is formed of melted solder pre-forms as describedbelow that fill a respective capacitor filter hole 108 extending betweenthe internally facing filter substrate side 94 and an externally facingfilter substrate side 96.

Each discoidal capacitive filter 100 comprises at least one capacitorelectrode formed within the filter substrate and extending outward froma filter array conductive path 110 in overlapping spaced relation to atleast one common ground plate. The number and dielectric thicknessspacing of the capacitor electrode sets varies in accordance with thecapacitance value and the voltage rating of the discoidal capacitor. Thecapacitor active and ground electrodes are formed of silver thick films,silver-palladium alloy thick films, or silver-platinum alloy thick filmsdisposed on inner capacitive filter layer surfaces during thefabrication of the capacitive filter array 90. The number of capacitoractive and ground electrodes, the sizes of each and the spacing andoverlapping relation can be varied for each discoidal capacitive filter100 within the capacitive filter array 90 and between differing modelsof such capacitive filter arrays 90 to tailor the filter characteristicsto the circuitry of the particular IMD. In the depicted example, thecapacitive filters 100 either have three capacitor active electrodes112, 114, 116 or two capacitor active electrodes 122, 124 that arespaced from three ground electrodes 102, 104, 106 that extend inwardfrom the filter substrate edge 98. In operation, the discoidal capacitorpermits passage of relatively low frequency electrical signals along theconductive path it is coupled with, while shielding anddecoupling/attenuating undesired interference signals of typically highfrequency.

The ground solder joint 130, preferably formed of solder or a conductiveepoxy, adheres against a metallized layer 111 formed on the filtersubstrate edge 98 as described below in reference to FIG. 12 thatelectrically connects the three capacitor ground electrodes 102, 104,106 together. The ground solder joint 130 also electrically connects thethree capacitor ground electrodes 102, 104, 106 to the ferrule 20through the conductive ground trace 132, the plurality of substrateground paths 118, and the substrate-ferrule braze joint 48. The groundsolder joint 130 can be formed of ABLEBOND.RTM.8700 electricallyconductive silver-filled epoxy adhesive provided by ABLESTIKLABORATORIES of Rancho Dominguez, Calif. Other suitable electricallyconductive glue or epoxy-based adhesives and other suitable materialsmay also be employed in the present invention to form the ground solderjoint 130. Such materials include gold or copper-filled epoxies, carbonor graphite-filled epoxies or even electrically conductive plasticsacting effectively as adhesive joints after their application and uponcooling, such as at least some of the electrically conductive plasticsor polymers disclosed in U.S. Pat. No. 5,685,632. The ground solderjoint 130 and the solder layers 128 mechanically join the externallyfacing filter substrate side to the internally facing substrate side.The solder layers 128 electrically join each filter array conductivepath 110 to a substrate conductor pad 82 of each substrate conductivepath 50.

The substrate-ferrule braze joint 48 is preferably formed of 99.9% orpurer gold or a nickel-gold alloy that adheres to the metallizationlayer 140 on substrate edge 46 and to the ferrule wall 22 and provides ahermetic seal of the ferrule 20 with the metal-ceramic substrate 40. Thesubstrate-ferrule braze joint 48 may also be formed of: (a) gold alloyscomprising gold and at least one of titanium, niobium, vanadium, nickel,molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium,indium, and alloys, mixtures and thereof; (b) copper-silver alloys,including copper-silver eutectic alloys, comprising copper and silverand optionally at least one of indium, titanium, tin, gallium,palladium, platinum, and alloys, mixtures and combinations thereof; and(c) silver-palladium-gallium alloys.

The filtered feedthrough assembly 10 thus provides a plurality ofminiaturized, electrically isolated, and capacitively filtered,electrical conductors formed of conductive path 50 and 110 extendingbetween a respective internal bonding pad 120 of the internally disposedportion 12 and bonding button 70 of the externally disposed portion 14when the feedthrough assembly 10 is affixed into an opening 204 in thecase 202 of the electronic device, e.g., the IMD 200 of FIGS. 8 and 9.The case 202 for an IMD is preferably fabricated of titanium, and theferrule flange 32 is welded thereto. The ferrule 20 is preferably formedof niobium because niobium has a comparable CTE to the CTE of AlO₂ whichis a preferred substrate ceramic material. However, the ferrule may beformed of titanium, titanium alloys such as titanium-6Al-4V ortitanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium,tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium,silver, and alloys, mixtures and combinations thereof.

FIG. 10 is a flow chart illustrating the steps of fabricating themulti-layer, co-fired metal-ceramic substrate 40 adapted to be brazedwith the capacitive filter array 90 formed in the steps of FIG. 11, theferrule 20, and other components in the steps of FIG. 12. In step S100the ceramic layers 52-60 are preferably tape cast from conventionalceramic or low temperature co-fired ceramic, such as alumina, aluminumnitride, beryllium oxide, silicon dioxide, etc., that has a CTEcompatible with the CTE of the material of the ferrule 20. Preferably,88%-96% pure alumina (AlO₂) is tape cast using conventional “greensheet” techniques on glass-ceramic or MYLAR support materials. Ingeneral, such techniques start with a ceramic slurry formed by mixing aceramic particulate, a thermoplastic polymer and solvents. This slurryis spread into ceramic sheets of predetermined thickness, typicallyabout 0.006-0.010 inches thick, from which the solvents are volitized,leaving self-supporting flexible green sheets.

In step S102, the holes that will be filled with conductive material toform the vias 62-68 of each conductive path 50 and the aligned groundvias, as well as the button cavities 80 of the layer 60 are made, usingany conventional technique, such as drilling, punching, laser cutting,etc., through each of the green sheets from which the ceramic layers52-60 are formed. The vias 42 may have a size appropriate for the pathspacing, with about a 0.004 inch diameter hole being appropriate for0.020 inch center to center path spacing.

In step S104, the via holes are filled with a paste of refractory metal,e.g., tungsten, molybdenum, or tantalum paste, preferably using screenprinting. In step S104, the conductive traces, e.g. traces 72, 74, 76,are also applied to particular surface areas of the ceramic layers 52-60over the vias. The traces may comprise an electrical conductor, such ascopper, aluminum, or a refractory metal paste, that may be deposited onthe green sheets using conventional techniques. The traces may bedeposited, sprayed, screened, dipped, plated, etc. onto the greensheets. The traces may have a center to center spacing as small as about0.020 inch (smaller spacing may be achievable as trace formingtechnology advances) so that a conductive path density of associatedvias and traces of up to 50 or more paths per inch may be achieved.

In these ways, the via holes are filled and the conductive traces areapplied to the green sheets before they are stacked and laminated instep S106 using a mechanical or hydraulic press for firing. The stackedand laminated ceramic layers are trimmed to the external edge dimensionssufficient to fit within the ferrule opening, taking into account anyshrinkage that may occur from co-firing of the stacked layers. In stepS110, the assembly of the stacked, laminated and trimmed green sheets isco-fired to drive off the resin and sinter the particulate together intoa multi-layer metal-ceramic substrate 40 of higher density than thegreen sheets forming the layers 52-60. The green sheets shrink inthickness when fired such that a 0.006 inch thick green sheet typicallyshrinks to a layer thickness of about 0.005 inch. The green sheets maybe fired using conventional techniques, with low temperature co-firedceramic techniques being recommended when copper or aluminum are used.

In step S112, the outer edge 46 and the inward and outward facingsubstrate surfaces 42 and 44 are machined and polished to size andfinish specifications. Then, in step S114, the various regions of theoutward facing surface 44 are metallized to form the button braze joints78 for each conductive path 50 and the band-shaped, ground plane layer136 electrically connecting all of the substrate ground paths 118together at the outward facing ends thereof. The substrate edge 46 isalso metallized with metallization layer 140. These metallization layersare preferably sputtered films of niobium, titanium, tungsten,molybdenum or alloys thereof. The machining and polishing of the outeredge 46 which is then metallized improves the dimensional tolerances ofthe co-fired substrate 40 which in turn enables the reliable use of thesubstrate-ferrule braze joint 48 that is formed in step S300 of FIG. 12.

In a preferred embodiment of the present invention, where pure gold isemployed to form the substrate-ferrule braze joint 48, a 25,000 Angstromthick layer of niobium is preferably sputtered onto substrate edge 46and on edge bands of the inward facing surface 44 to form theband-shaped, ground plane or trace layers 132 and 136 by vacuumdeposition using a Model No. 2400 PERKINELMER.RTM. sputtering system.The niobium layer is most preferably between about 15,000 and about32,000 Angstroms thick. These metallization layers may not be requiredif metals such as: (i) gold alloys comprising gold and at least one oftitanium, niobium, vanadium, nickel, molybdenum, platinum, palladium,ruthenium, silver, rhodium, osmium, iridium., and alloys, mixtures andthereof; (ii) copper-silver alloys, including copper-silver eutecticalloys, comprising copper and silver and optionally at least one ofindium, titanium, tin, gallium, palladium, platinum; or (iii) alloys,mixtures or combinations of (i) or (ii) are employed for thesubstrate-ferrule braze joint 48.

FIG. 11 is a flow chart illustrating the steps of fabricating thecapacitive filter array 90 adapted to be brazed with the co-firedmetal-ceramic substrate 40 formed in the steps of FIG. 10, the ferrule20, and other components in the steps of FIG. 12. The capacitive filterarray 90 is also formed of layers of ceramic material, preferably bariumtitanate, and screen printed, conductive, capacitor active and groundelectrodes that are co-fired to form a monolithic structure.

In step S200, the barium titanate ceramic layers are tape cast, and thecapacitor active and ground electrodes are screen printed on thesurfaces thereof in step S202. The capacitor electrodes are formed ofsilver thick films, silver-palladium alloy thick films, orsilver-platinum alloy, thick films. The layers are stacked and laminatedusing a mechanical or hydraulic press in step S204, and the stacked andlaminated layers are machined and drilled to form the capacitorconductive path receiving, capacitive filter holes 108 in steps S206 andS208.

The partly completed capacitor filter array 90 is fired in step S210 toform the monolithic structure. Then, in steps S212 and S214, the edgesof the active capacitive filter electrodes 112, 114, 116 or 122, 124exposed by the capacitor holes 108 and the capacitive filter groundelectrodes 102, 104 and 106 are coupled together electrically in commonor “terminated”. A conductive metal frit that contains one of silver,palladium, platinum, gold and nickel alloys thereof, is placed in thecapacitor holes 118 and along the array side 98 and melted to form thetermination layers 109 and 111 shown in FIGS. 5 and 6. Most commonly,the conductive frit comprises one of silver, silver-palladium alloy ornickel-gold alloy. Alternatively, the capacitor holes 118 and the arrayside 98 may be electroplated with layers of nickel and gold. In stepS212, the capacitor filter array 90 is fired again to densify thetermination layer materials.

FIG. 12 is a flow chart illustrating the steps of fabricating thefiltered feedthrough assembly 10 from the capacitive filter array 90formed in the steps of FIG. 11, the co-fired metal-ceramic substrate 40formed in the steps of FIG. 10, the ferrule 20, and other components.First, the metal-ceramic substrate 40 is fitted into the ferrule openingand hermetically sealed thereto using the gold or gold alloysubstrate-ferrule braze joint 48 and the externally disposed contactbuttons 70 are sealed into the button cavities 80. Then, the capacitorfilter array 90 is attached to the interior facing surface of themetal-ceramic substrate 40, the capacitor filter conductive paths 110fill the filter holes 108 using reflow solder techniques, and theinternally disposed ground solder joint 130 and the plurality ofinterior contact pads 120 are attached.

In step S300, the ferrule 20, braze preforms that melt to form thesubstrate-ferrule braze joint 48, the metal-ceramic substrate 40, andthe externally disposed contact buttons 70 in the button cavities 80 arestacked into a braze fixture. Advantageously, these components that areassembled together in step S300 self center and support one another inthe braze fixture. This improves the ease of manufacturing and increasesmanufacturing batch yields. The stacked assembly is subjected to brazingtemperatures in a vacuum or inert gas furnace in step S302, whereby thebraze preforms melt to form the substrate-ferrule braze joint 48 and thebuttons 70 fill the button cavities 80 and adhere to the braze joints78. As the assembly cools, the ferrule contracts more than the co-firedsubstrate, which puts the co-fired substrate in a state of compression.

In step S304, the conductive plated pads 82 and the band-shaped, groundplane or trace layer 132 electrically connecting all of the substrateground paths 118 together at each inward facing end thereof are adheredonto the surface 42 as metallization layers. Each metallization layerpreferably comprises sputtered films, first of titanium, then of nickel,and finally of gold, so that a three film metallization layer is formedin each case.

In step S306, the discoid capacitive filter array 90, reflow solder, andthe interior contact pads 120 are assembled onto the inward facingsurfaces of the sub-assembly formed in step S304, and these componentsare heated in step S308. The heating causes the solder to flow into andfill the capacitive filter conductive path holes 108 to complete theformation of the capacitive filter conductive paths 110 and the solderpads 128 shown in FIG. 7 and to adhere the internally disposed bondingpad 120. The solder may be an indium-lead or tin-lead alloy, and theinternally disposed bonding pads 120 may be formed of Kovar alloy platedwith successive layers of nickel and gold. The final layer that isexposed to air and that lead wires are bonded or welded to as shown inFIG. 9 preferably is gold.

In step S310, the ground solder joint 130 is molded around and againstthe filter substrate edge 98 and the band-shaped, ground plane or tracelayer 132. The ground solder joint 130 electrically connects the threeground electrodes 102, 104, 106 together and to the ferrule 20 throughthe plurality of substrate ground paths 118 and the substrate-ferrulebraze joint 48. The ground solder joint 130 also mechanically bonds thediscoid capacitive filter array 90 with the multi-layer metal-ceramicsubstrate 40. Since the ground solder joint 130 does not need to providea hermetic seal, it may be formed of a number of materials as describedabove.

In the sputtering steps of the present invention, a DC magnetronsputtering technique is preferred, but RF sputtering techniques may lesspreferably be employed. A DC magnetron machine that may find applicationin the present invention is an Model 2011 DC magnetron sputtering devicemanufactured by ADVANCED ENERGY of Fort Collins, Colo.

The pin-to-pin spacing of two single pin or unipolar feedthroughs istypically on the order of 0.125 inches. The above-described capacitivefiltered feedthrough array provides a spacing of 0.050 inches betweenadjacent conductive paths. The feedthrough assembly 10 can be formedproviding the nine capacitively filter array conductive paths within aferrule 20 that is 0.563 inches long and 0.158 inches wide.

While the present invention has been illustrated and described withparticularity in terms of a preferred embodiment, it should beunderstood that no limitation of the scope of the invention is intendedthereby. The scope of the invention is defined only by the claimsappended hereto. It should also be understood that variations of theparticular embodiment described herein incorporating the principles ofthe present invention will occur to those of ordinary skill in the artand yet be within the scope of the appended claims.

What is claimed is:
 1. A method of manufacturing a filtered feedthroughassembly adapted to be fitted into an opening of a case of ahermetically sealed electronic device, the feedthrough assembly havingan internally disposed portion configured to be disposed inside the caseand an externally disposed portion configured to be disposed outside thecase, the method comprising: providing an electrically conductiveferrule having a ferrule wall adapted to be fitted into the case openingwith an inner wall surface defining a centrally disposed ferrule openingand extending between opposed internally and externally facing ferrulesides; forming a multi-layer, co-fired metal-ceramic substrate havingopposed internally facing and externally facing substrate surfacesjoined by a common substrate edge, the step of forming the metal-ceramicsubstrate further comprising: forming a plurality of metal-ceramicsubstrate layers having internally and externally facing layer surfaces;forming a plurality of substrate conductive paths extending through theco-fired metal-ceramic substrate between the internally and externallyfacing substrate surfaces and electrically isolated from one another;and forming a further plurality of substrate ground paths extendingthrough the co-fired metal-ceramic substrate between the internally andexternally facing substrate surfaces and electrically isolated from thesubstrate conductive paths; hermetically sealing the common substrateedge to the ferrule inner wall within the centrally disposed ferruleopening and electrically coupling the plurality of substrate groundpaths to the ferrule; forming a discoidal capacitive filter array formedof a ceramic capacitive filter substrate having an internally facingfilter substrate side and an externally facing filter substrate sidejoined by a common filter substrate edge, the step of forming capacitivefilter army substrate further comprising the steps of: forming aplurality of filter array conductive paths electrically isolated fromone another and extending between the internally facing filter substrateside and the externally facing filter substrate side; forming aplurality of discoidal capacitor filters each comprising at least onecapacitor active electrode formed within the filter substrate andextending outward from a filter array conductive path and a commoncapacitor ground electrode; mechanically joining the externally facingfilter substrate side to the internally facing substrate sideelectrically connecting each filter array conductive path to a substrateconductive path; and electrically connecting the common capacitor groundelectrode of the discoidal capacitor filters to the plurality ofsubstrate ground paths; whereby the filtered feedthrough assemblyprovides a plurality of miniaturized, electrically isolated, andcapacitively filtered, feedthrough conductive paths with low inductanceeach comprising a substrate conductive path joined to a filter arrayconductive path and extending between the internally disposed portionand the externally disposed portion when the feedthrough assembly isaffixed into an opening in the case of the electronic device.
 2. Themethod of claim 1, wherein the case of the electronic device and theferrule is formed of metallic materials amenable to being weldedtogether, the electrically conductive ferrule is further formed with awelding flange extending outwardly of the ferrule wall away from theferrule opening for a predetermined distance defining a flange width,and the welding flange formed with a stress relieving thickness forabsorbing stress caused by thermal welding energy applied to the ferruleand flange in the process of welding the flange to the case.
 3. Themethod of claim 2, wherein the co-fired metal-ceramic substrate ceramicmaterial is formed of one of the group consisting of alumina, aluminumnitride, beryllium oxide, and silicon dioxide that has a coefficient ofthermal expansion compatible with the coefficient of thermal expansionof the material of the ferrule.
 4. The method of claim 3, wherein theferrule is formed of a conductive material selected from the groupconsisting of niobium, titanium, titanium alloys such as titanium-6Al-4Vor titanium-vanadium, platinum, molybdenum, zirconium, tantalum,vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium,palladium, silver, and alloys, mixtures and combinations thereof.
 5. Themethod of claim 1, wherein the metal-ceramic substrate is formed by thefurther method of: forming a plurality of planar ceramic layers shapedin a green state to have a layer thickness; forming a plurality ofsubstrate conductive path via holes and ground path via holes extendingtherethrough between an internally facing layer surface and anexternally facing layer surface; filling conductive path and ground pathvia holes with conductive material; forming conductive path traces andground traces on selected ones of the internally and externally facinglayer surfaces; assembling and laminating the plurality of ceramiclayers together; punching the outer dimensions of the assembled andlaminated plurality of ceramic layers; co-firing the assembled andlaminated ceramic layers from the green state to form the substratehaving the opposed internally facing and externally facing substratesides joined by a common substrate edge; and machining and polishing thecommon substrate edge to enable to enable electrical connection of theground vias in common and to the ferrule in the step of hermeticallysealing the common substrate edge to the ferrule inner wall within thecentrally disposed ferrule opening.
 6. The method of claim 5, whereinthe further method of hermetically sealing the common substrate edge tothe ferrule inner wall within the centrally disposed ferrule or openingand electrically coupling the plurality of substrate ground paths to theferrule comprises forming a substrate-ferrule braze joint of aconductive braze material.
 7. The method of claim 5, wherein theplurality of substrate conductive paths extending through the co-firedmetal-ceramic substrate between the internally and externally facinglayer surfaces are formed by the further steps comprising: placing asubstrate conductor braze pad on the substrate internally facing sideover an exposed conductive trace or via of each conductive path; placinga bonding button over each exposed braze pad; and applying heat to meltthe braze pads and to thereby mechanically and electrically join thebonding button with the substrate conductive path.
 8. The method ofclaim 7, wherein the externally disposed bonding buttons are formed of aconductive material selected from the group consisting of niobium,platinum or a platinum-iridium alloy, titanium, titanium alloys such astitanium-6Al-4V or titanium-vanadium, molybdenum, zirconium, tantalum,vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium,palladium, silver, and alloys, mixtures and combinations thereof.
 9. Themethod of claim 5, further comprising: in the green state and prior toassembling and laminating the ceramic layers, forming a plurality ofspaced apart button cavities extending through the externally facingceramic layer of the metal-ceramic substrate located to be aligned withthe plurality of substrate conductor paths through the remaining ceramiclayers; and wherein the plurality of substrate conductive pathsextending through the co-fired metal-ceramic substrate between theinternally and externally facing layer surfaces are formed by thefurther method comprising: placing a substrate conductor braze padwithin each button cavity of the substrate internally facing side andover an exposed conductive trace or via of each conductive path; placinga bonding button in each button cavity over each braze pad; and applyingheat to melt the braze pads and to thereby mechanically and electricallyjoin the bonding button with the substrate conductive path.
 10. Themethod of claim 5, wherein a conductive paste material selected from thegroup consisting of copper, tungsten, molybdenum and gold is used in thesteps of filling conductive path and ground path via holes and formingconductive path traces and ground traces and is applied by screenprinting to said ceramic layers in the green state.
 11. The method ofclaim 5, wherein: the capacitive filter array is formed through thefurther method of extending a filter array hole between the internallyfacing filter substrate side and the externally facing filter substrateside and through at least one capacitor active electrode; and the methodof mechanically joining the externally facing filter substrate side tothe internally facing metal-ceramic substrate side and electricallyjoining each filter array conductive path to a substrate conductive pathcomprises the steps of: placing a substrate conductor braze pad on theexternally facing metal-ceramic substrate side over an exposedconductive trace or via of each conductive path; placing the internallyfacing filter substrate side against the externally facing metal-ceramicsubstrate side with the filter array holes aligned with the substrateconductor braze pads on the externally facing metal-ceramic substrateside placing solder material in the filter array holes to form asub-assembly; and heating the sub-assembly to reflow solder fill thefilter array holes and mechanically bond with the substrate conductorbraze pads on the substrate internally facing side, whereby the reflowsolder within each filter array hole forms at least part of a filterarray conductive path.
 12. The method of claim 11, further comprisingthe method of placing an internally disposed bonding pad against thesolder placed in each of the plurality filter array holes whereby theplurality of internally disposed bonding pads adhere to the reflowsolder filling the plurality of filter array holes on the internallyfacing side of the capacitive filter array.
 13. The method of claim 12,wherein the internally disposed bonding pads are formed of a conductivematerial selected from the group consisting of copper, nickel, gold andaluminum and alloys, mixtures and combinations thereof.
 14. The methodof claim 11, wherein the capacitive filter array is formed through thefurther method of: forming a plurality of capacitor active electrodesthe filter substrate and extending outward from a filter arrayconductive path, applying a hole metallization layer within the holeelectrically coupling the capacitor active electrodes together; forminga further plurality of capacitor ground electrodes formed within thefilter substrate and extending inward from the filter substrate edge,applying an edge metallization layer overlying the filter substrate edgeelectrically coupling the capacitor ground electrodes together, wherebyeach filter array hole is formed to extend between the internally facingfilter substrate side and the externally facing filter substrate sideand through the plurality of capacitor active electrodes.
 15. The methodof claim 1, wherein the method of mechanically joining the externallyfacing filter substrate side to the internally facing substrate side andelectrically joining each filter array conductive path to a substrateconductive path comprises reflow soldering the filter array conductivepath with the substrate conductive path.
 16. The method of claim 1,further comprising a further method of: providing a plurality ofinternally disposed bonding pads supported along the internally facingfilter substrate side, each internally disposed bonding pad electricallyconducted with a filter array conductive path of the capacitive filterarray; and providing a plurality of externally disposed bonding buttonssupported along the externally facing metal-ceramic substrate side, eachexternally disposed bonding button electrically conducted with asubstrate conductive path.
 17. The method of claim 16, wherein theinternally disposed bonding pads are formed of a conductive materialselected from the group consisting of copper, nickel, gold and aluminumand alloys, mixtures and combinations thereof.
 18. The method of claim16, wherein the externally disposed bonding buttons are formed of aconductive material selected from the group consisting of niobium,platinum or a platinum-iridium alloy, titanium, titanium alloys such astitanium-6Al-4V or titanium-vanadium, molybdenum, zirconium, tantalum,vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium,palladium, silver, and alloys, mixtures and combinations thereof.